Why has evolution not created yet another platypus?

Is it possible to predict evolution or is it acting randomly?

With a sniff, wading through the undergrowth, a small shaggy creature wanders through the forest at night, nuzzling now and then in one place and now in another, seeking out the smell of its soft-bodied dinner. In the forest it is dark, the sight of this creature is poor, but long whiskers and a good sense of smell allow it to navigate. In the event of a threat, it is capable of developing dizzying speed, rushing through vegetation, diving into burrows, and quickly disappearing from sight.



Completely unoriginal lifestyle. Many animals stroll through the woods at night, and in a similar way look for small prey: hedgehogs, shrews, weasels, and besides them large animals - opossums and even pigs. The world is full of such animals.

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But this animal is not like that. All the others are hairy. The coat of this animal is also soft, consisting of millions of thin strands. But this is not hair. All the others move on four legs and bear living offspring. But not this.



Scratching, studying the environment, sniffing, this animal sometimes creates a duet with its couple, calling to each other, staying in touch while passing through its territory. The cry of the male gives out its name: "Kii-vii, kii-vii".





Filters that separate food from water, whales and whale sharks have a completely different structure.



We are in New Zealand, and this night insectivorous is a bird, with stumps instead of wings, a cat-like mustache, soft feathers, and, unlike other birds, with nostrils located at the tip of the beak. Many people call it "honorary mammal".



New Zealand is crammed with unusual creatures. But what is still unusual is what is not there: mammals. On the islands there is hardly a shred of wool. Apart from the fur seals filling the beautiful beaches of New Zealand, the only three local mammals are the three bats - and even they are strange.



On the other half of the globe, in Cuba, there are some oddities. An owl as tall as a first-grader who, among other things, fed on young giant ground-sloths, unfortunately died out (like giant sloths comparable in size to a gorilla), but a hummingbird the size of a bumblebee still lives on the island; chickle-toothed, an archaic mammal, as if descended from the pages of children's books, with poisonous saliva and a long, flexible, whiskered bear; something like a guinea pig the size of a beagle that can climb trees and produce an abundant green poop in the shape of a banana.



Even small islands have their own unusual wonders. On the island of Lord Howe with an area of ​​14.5 sq. Km. in the form of a crescent, located in the Tasman Sea, live "tree lobsters", which, despite the name, are obese and hefty members of the insect family, usually characterized by thin, rod-like representatives. There is a lizard in the south of the Pacific Ocean in the Solomon Islands, posing as a monkey: a giant tentacle skink is a shiny, slender, 70 cm lizard with a tenacious tail, with which it clings to tree branches in search of fruit. And everyone has heard of the once-existing dodo bird from the island of Mauritius in the Indian Ocean. She did not know how to fly, had no feathers, ate fruit, was about the size of a turkey, reached a meter in height, and weighed up to 20 kg.



Among the small islands, the Hawaiian Islands occupy the first place in the number of oddities: the dragonflies, whose larvae, usually aquatic, live on the ground; voracious carnivorous caterpillars; fruit flies, switching from fruit to decaying plants; one more fruit flies with a hammer head that protect their territory, butting like rams. The flora of Hawaii is just as strange - the brigamy holds a remarkable championship, looking “like a pin, topped with a lettuce salad”.



And then there is Madagascar, which is sometimes called the eighth continent due to its distinctive flora and fauna. There is a pygmy hippo; adaptive radiation of lemurs (including one weighing 35 kg, hanging from branches, like sloths, and the other, like an overgrown koala - but both of these species were exterminated by humans); three-meter 500-pound elephant birds (extinct; the heaviest of all birds that have ever existed); half of all the world chameleon species that shoot tongues that are twice the length of their body; fossil frogs the size of a large pizza; vegetarian crocodiles; beetles with a neck, like a giraffe. The plants of Madagascar are no less strange, including desert forests, consisting of tall and thin stems, studded with thorns, and thick baobab trees that look as if they were stuck in the ground upside down.



Last in order, but not least, are the wonders of Australia: the platypus, the kangaroo, the koalas - in the rest of the world there is nothing like it.





The flight of birds and bats developed independently. Both those, and others adapted forelegs under wings, only birds use feathers, and bats - skin.



What it all boils down to: The islands give us an idea of ​​the alternative worlds of evolution that could exist if life had turned somehow differently. What if mammals died out at the end of the Cretaceous, along with dinosaurs? New Zealand demonstrates how it might look. Where would the primates evolve if monkeys had not evolved? Pay attention to the lemurs that are found exclusively in Madagascar.



The islands give us a recipe for evolution. The resulting dishes tell us that it is impossible to predict what will happen in the stove. Change the ingredients or the sequence of their addition, add heat, do not put something, throw one pinch of salt instead of two - and the result may be completely different. The island recipe book is replete with examples of chance and contingency, and the diversity of results suggests that it is very difficult to predict what will appear on any of the islands as a result of evolution.



For many decades, the generally accepted view of evolutionary biology, as formulated by Stephen Jay Gould, was one of these accidents: change any event in the history of life, and all life can be completely different. The existence of such a modern life as it is is not inevitable and not even the most likely - all this is just a game of chance.



But in recent years, the backbone of scientists has emerged, under the leadership of Saiman Kovney Morris, who took the opposite point of view - they argue that certain evolutionary solutions are quite likely. In the course of evolution, completely different species constantly develop similar adaptive solutions to problems encountered in their environment - for example, a very similar structure of the eyes of people and octopuses. These repeated solutions are called evolutionary convergence. From this point of view, randomness does not have much influence on history, and their effects are erased by the predictable pressure of natural selection.



We can easily understand convergence — species that adapt equally to the same situations. But what about evolutionary solutions, unique in their own way? Why have other species not developed similar solutions in the course of evolution?



One explanation for the evolution exceptions is the appearance of unusual species in unique environmental conditions. Perhaps they did not have analogues because no one fell into similar circumstances. This may explain the koala. Her whole life is tied to eucalyptus trees and eating their leaves, which contain many poisons. As a result, the koala’s digestive system is extremely long, which gives it plenty of time to detoxify the leaves and extract nutrients. The slow passage of food in combination with its low nutritional value means that the koala has to minimize energy consumption and sleep most of the day. Eucalyptus trees come from Australia, so it is possible that the originality of the koala reflects the uniqueness of its environment.



But it seems to me that in most cases this explanation does not fit. Platypuses are found in streams, ponds, lakes and rivers of eastern Australia, where they can eat river crayfish and other aquatic invertebrates, which they search for, darting along the bottom, and feeling the victim with the help of electroreceptors located on their beaks. The rest of the time, they are retired to their chambers at the ends of long holes dug down on the river bank. This lifestyle seems possible in many places other than Australia. The streams they inhabit are very similar to the stream that passed behind my friend's house when we grew up in St. Louis. North America is full of brooks where crayfish live, many of which are located in climates similar to the one in which the platypus lives, with no more terrible predators than those living in Australia. So where is our duckbill twin? Why somewhere else did not appear anything like? Or kangaroo, or any other of the examples I have listed - they all live in habitats that are found elsewhere.



Other explanations for evolutionary exceptions are that natural selection is either not as predictable or not as powerful as some believe. That is, even living creatures that live in similar conditions can evolve in different ways.



The main reason for the lack of convergence is the presence of several ways to adapt to the tasks posed by the environment. Think about how vertebrates float. Many use their tail, but the tails are all different. In fish, the tails are flattened vertically and move left and right. Crocodiles swim the same way. But whales have tails flattened out horizontally and move up and down. Other animals, such as snakes or sea snakes, make wave-like movements with their whole bodies. Some birds, such as cormorants and loons, can quickly move under water, furiously rowing with webbed feet. On the other hand, some species swim with the help of changed forelimbs - sea lions have such flippers, and penguins use wings for this. However, the most amazing swimmer can be a tree sloth, whose long forelimbs, which have evolved to hang from branches, help him crawl. In invertebrates, there are even more solutions for movement in the water, for example, reactive movement in octopuses and squid.



This list of different methods of rapid movement in water raises a natural question: how similar should the two properties be in order to consider them convergence? Squids and dolphins use a very different anatomical system for rapid movement in the water - they clearly do not converge. And another dissimilar method of movement is rowing with webbed feet in some aquatic bird species.



Other examples are not so obvious. What about the flat tails of whales and sharks - they are similar in structure and work, only one of them is vertical, and moves left and right, and the other is horizontal, and moves up and down? Are these features small variations in convergence or non-convergent solutions with similar functionality? I suspect that most people will consider horizontal and vertical flat tails essentially one solution.



Step back to the properties that lead to similar functional results, but showing great anatomical diversity between species. Among vertebrates, active flight appeared in the process of evolution three times: in bats, in birds and in pterosaurs (large reptiles that conquered the skies in the age of dinosaurs). They all changed their forelimbs to a state of wings, and fly (or flew, in the case of pterosaurs) in essence, in the same way, swinging the light structure down to produce lift and acceleration forward.



But a careful study shows that the wings of these flying vertebrates are arranged very differently. The most obvious difference lies in the aerodynamic surface itself. Birds use feathers that grow separately from the bones of the arms. The wing profile of bats and pterosaurs consists of thin, but very strong skin, stretched between the bones of the fingers and the body, and in some cases even connecting to the hind legs. The anatomy of the skeletons of the wings in these three groups are also very different.



So, are the wings that grew from the forelimbs of birds, bats, and pterosaurs, convergent adaptations for active flight, constructed differently? Or do they represent alternative, non-convergent ways to develop active flight in the process of evolution?



One more example. The largest fish in the sea is the 18-meter whale shark. Like whales, it feeds through a filter, swallowing huge amounts of water with its massive mouth and filtering out tiny food. But this is where the similarities end. Baleen whales — blue, hunchbacked, gray, and others — catch prey by pushing water through rigid plates like a comb of whalebone, forming a curtain hanging from the upper jaw. Any part of the food is larger than the gap in the whisker, is delayed by its inner surface, and then digested. Conversely, in whale sharks, water passes through gill slits located on the sides of the back of the head. Cartilaginous filters are arranged so that water passes between the filters, through the gills and out into the ocean, and food particles continue to move past the gill crevices and form a mass in the throat, which is then swallowed. So baleen whales and whale sharks are large aquatic creatures that use huge mouths to draw water and filter small prey. But the specific structure of the filters differs in design, layout and pattern of operation. Convergent is adaptations for filtered food or not?



It is possible to arbitrarily draw a line between convergence and its absence for structures that largely coincide and lead to similar functional advantages. I tend to think of the wings of birds, bats, and pterosaurs as convergence. In the same way, I believe that baleen whales and whale sharks are convergent, since they have large mouths and feed on plankton. However, I myself consider their filtration and power systems not as convergent, but as alternative adaptations to such nutrition. But in such cases there are no right or wrong answers.





Cheetahs and hyenoid dogs hunt the same animals using different strategies and anatomical adaptations.



In other cases, species can adapt, evolving in distinctly different ways, producing non-convergent phenotypes with similar functionalities. My favorite example of this phenomenon is related to the underground life of rodents. More than 250 species from the rat clan spend most of their lives underground, moving through independently dug tunnels. Such digging holes in the process of evolution constantly appeared in rodents, but was achieved by various methods. Many rodents dig burrows in the usual way, loosening the ground in front of themselves with the help of the forelimbs, and throwing it back. The forelimbs of such species are strong and muscular; claws long and strong. Other species use teeth instead of claws to remove soil. As expected, their teeth are long and prominent, even by the standards of rodents, and the muscles of the jaws and skull are massive. Most dental excavators get rid of the soil, pushing it back with the front limbs, but some rodents have another variation - they tamp the softened soil into the walls of the tunnel with the help of a long, shovel-like muzzle. The differences in the anatomy of these excavators are a clear illustration of non-convergent adaptations leading to similar functional outcomes.



Convergence may not occur for another reason. Often there are several different functional ways to adapt to the environment. For example, look at how species that are potential prey for predators can adapt to the presence of a predator like a lion. One option - to develop in the process of evolution running opportunities to overtake them, but there are other options. This is camouflage, passive protection or active protection. The resulting adaptations will undoubtedly be non-convergent, for example, African buffalo horns, armor-bearing underwear and turtles, long impala legs, porcupine spikes, poison and spitiness accuracy of spitting cobra and the motley skin of a forest antelope.



Many solutions to the same problem are not limited to protection. Cheetahs and hyena-like dogs hunt for the same animals, but the cat does it with short throws at tremendous speed, and the dogs run slower, but longer, exhausting the victim. And their adaptation varies accordingly: very long legs and a flexible cheetah's spine allows it to accelerate to 110 km / h; Hyena dogs' excellent endurance allows them to run at a constant speed of 50 km / h long enough to tire the victim (and cheetahs can run at high speed only for short distances).



Or consider the adaptation of animals in order to obtain nectar. Plants often produce a sweet-smelling, sweet liquid to lure insects, birds, and other animals to help with the reproductive process. When an animal sticks its head or the whole body into a flower to feast on nectar, it becomes covered with pollen.When moving to the next flower, part of the pollen disappears and pollinates the ovules of the plant.



Many flowers have long tubules with nectar at the bottom — in this way the flower limits the access and receipt of pollen to several specific species that are well adapted for use by this flower, for example, moths with long proboscis and hummingbirds with equally long beaks and tongues. Such species, due to adaptation, do not often visit other flowers, which limits the likelihood that the pollen from them will fall into a flower of another species and disappear in this way.



But not all nectar-eating creatures play by the rules. Some species of insects, birds and mammals gnaw through a hole in the base of the flower, bypassing the petals and their pollen, thus not fulfilling their role in the co-evolutionary deal. To do this, these nectarine thieves use very different adaptations. They do not need long tongues and parts of the head to get to the bottom of long tubes, they develop properties that improve their ability to break through the material of the flower. Some hummingbirds have barbs on their beaks for this purpose. The bird hook hook on the end of the upper part of the beak has a hook used to cut flowers. In these many examples, it is clear that there are often several evolutionary options for solving the problems posed by the environment. But the fact that there are many of them does not mean that all options will appear as a result of evolution.Conway Morris and his team say that usually one option has an advantage over others, and therefore the same properties converge again and again. However, convergence does not always appear. Why should natural selection not use the same property every time?



It may happen that two or more properties turn out to be equivalent. Camouflage or the ability to escape at high speed can be equally successful ways to avoid predators. Or one method will be more successful than another for a specific purpose, but with other disadvantages that outweigh its advantages. A quick escape from an approaching predator can be a good way to escape, but camouflage can improve the abilities of animals such as snakes to trap their own prey. When survival and reproduction are summed up, individuals with camouflage can be just as successful as those that rely on speed, and they, too, due to reproduction pass on their genes to the next generation. As a result, natural selection does not prefer one to the other. The appearance of properties may be a matter of chance, a question of which mutation will arise first,when the hunt begins on individuals.



Conversely, the evolution of a property may depend on the initial phenotype and genotype of the species. In general, the active species may be predisposed to develop properties that affect the increase in speed when a predator appears, and less mobile species may develop camouflage. None of the options prevails over the other, but the evolutionary result can strongly depend on the initial conditions.



It may be that one solution would be preferable, but in some cases it is easier to work out the most optimal solution. The French scientist Francois Jacob, who won the Nobel Prize for his research on DNA, proposed an analogy explaining why natural selection does not always lead to the appearance of a perfectly engineered organism. Jacob says that natural selection is not like an engineer designing the optimal solution for an existing problem. Better imagine a self-made master of all trades using the materials he has on hand to create a viable solution - not the best possible, but the best available under the circumstances.



Imagine the birds caught by the lake, full of slowly swimming fish. They can begin to dive for food, and eventually adapt to the aquatic lifestyle, producing large and powerful hind legs, like a cormorant, or changing the shape of their wings and bringing them closer to flippers, like penguins. Suppose that the best way to swim quickly and deftly is to move the strong muscular tail in the water, swinging it left and right, or up and down - this is what the fastest swimmers do. But birds do not have long tails - they lost them at the beginning of evolutionary history, more than a hundred million years ago, and they have only a small remnant of accrete bones (the “tails” of birds consist only of feathers, but not of bones). I’m not saying that re-working out a long tail as a result of evolution is impossible - but natural selection, a jack of all trades,most likely will not go this way. The birds already have wings and legs capable of providing driving force. It seems more likely that natural selection will work to improve the swimming functions of already existing structures than that it will develop a new structure from scratch, even if a new bird with a bone tail - which looks like a hybrid between a loon and a crocodile - will swim much better. .



But still, if the crocodile bird is better adapted - it will be the best swimmer - why don't waterfowl evolve in this direction? It is possible, sometimes in some direction it is impossible to pass: it is difficult to pass the evolution from one adaptive form to another because intermediate conditions will be unsuitable. A long, powerful tail is good for fast swimming, but a short tail can only get in the way and reduce swimming speed. Natural selection does not have foresight - it will not play in favor of a harmful property just because it is the first step on the path leading to excellence. In order for a property to emerge as a result of natural selection, each step on the path must be an improvement over the previous one - natural selection will never prefer deterioration, even if it is only a transitional evolutionary phase.



What do we get? Is convergence a fundamental force, a demonstration of the structure of the biological world, guided by the predictable influence of natural selection on the way to environmental outcomes predicted by the environment? Or, examples of convergent evolution are exceptions, specially selected illustrations of biological predictability in a random world in which most species do not have evolutionary parallels?



On this subject, you can argue hoarse. I will give the example of the platypus, you give a counterexample in the form of convergent hedgehogs; I will answer with a unique tree sloth; You will answer with a mouse jumping on two legs, independently of which appeared as a result of evolution on three continents. This is how this controversy developed historically - with the help of lists and storytelling.



We have to praise Conway Morris and his colleagues for pulling convergent evolution to the fore. Convergence was known to all of us as an artful trick of natural history, a vivid example of the possibilities of natural selection. But Conway Morris and his colleagues clearly showed that evolutionary copying occurs much, much more often than we thought. Now we understand that it is very often found in nature, and you can find its full examples. And yet it is not omnipresent.



It seems that just as often, and perhaps more often, species living in similar conditions do not adapt convergently. Now we need to move from describing historical patterns and collecting examples. We need to ask whether we can understand why convergence occurs in some cases and not in others - which explains to what extent convergence can occur, and to what extent it cannot, why rodents jumping on two legs independently appeared in the deserts of the whole world, and kangaroos only once. And for this we need not just add a few additional examples to the list. We need to test the hypothesis of evolutionary determinism directly.



Evolutionary biology was late in the experimental game — the legendary slowness of evolution prevented the development of experimental ideas. Now we know that this view is wrong, that evolution can go very quickly. And this understanding opens up new possibilities in studying evolution.



Jonathan Losos is a professor of biology, director of the Los-hos Laboratory at Harvard University, curator of the herpetological department of the Harvard Museum of Comparative Zoology. The author of the book "Lizards in the tree of evolution: ecology and adaptive radiation of anoles." An excerpt from the book "Unlikely Fate: Fate, Accident, and the Future of Evolution."